KR19990083518A - Process for joining two solid bodies and the resultant structural element - Google Patents

Abstract

The present invention relates to a method of joining two solid bodies, in particular two solid bodies of silicon, by a substantially smooth surface first coated with a monomolecular layer of sulfur containing organosilane and Obtained therefrom, it relates to a solid that can be used in the field of microelectronics or micromechanics.

Description

Process for joining two solid bodies and the resultant structural element

The present invention relates to a process for joining two solids through a substantially smooth surface first coated with a monomolecular layer of sulfur containing organosilane and to a composite solid obtained therefrom.

It is known that sufficiently smooth and clean solid surface surfaces can be bonded to one another at room temperature by adhesion. The initial weak interaction is enhanced so that the solids can no longer be separated from each other by heat treatments carried out at temperatures of 800 to 1100 ° C. when silicon solids are used. Such methods, described in the literature as "wafer bonding", "direct bonding" or "fusion bonding", have held important positions in both microelectronics and microsystems.

Comments on prior developments in the art can be found in the paper "History and Future of Semiconducter Wafer Bonding", U. Gosele, H. Stenzel, M. Reiche, T. Martini, H. Steinkirchner und Q.-Y. Tong in "Solid State Phenomena" 47 & 48, pp. 33 to 44 (1995).

Wafer bonding is not limited to bonding silicon solids. Many different materials can be bonded together in this manner. However, among other factors, problems arise from the high temperatures necessary to obtain acceptable bond strengths. As a result, structural elements containing thermosensitive structures as well as combinations of materials with different thermal expansion coefficients of the respective components cannot currently be sufficiently strong bonded.

A method has also been proposed in which wafer bonding of silicon wafers can be performed at room temperature. However, the process needs to be free of adsorbates on the surface and can only be achieved in ultra-high vacuum. The necessary cleanliness in this regard can be achieved by heat treatment at high temperatures, but this means that the process is also unsuitable for the thermosensitive structure, and another method is to use a plasma etching method. This method also involves heating the wafer. Although high stress can be obtained by the method carried out at room temperature, it is no longer possible to separate the bonded structure by this method.

This type of method is described for example in German Patent No. 44 09 931. In the present specification, as the prior art, first of all, the surface of the silicon wafer is first highly polished, and treated with a mixture of H ₂ SO ₄ and H ₂ O ₂ solution (to generate high density hydroxide groups) to prepare the surface of the wafer. A method for direct bonding of silicon wafers is described that makes it hydrophilic and then contacts water closely or closely at room temperature while water is present between them. The wafer is then heat treated to remove water between the wafers by reaction with silicon and the wafers are bonded by silicon-oxygen covalent bonds.

Furthermore, in some variations of the invention according to German Patent No. 44 09 931, water molecules are applied to the surface of the wafer to be bonded, but only to form hydroxide groups on these surfaces. Subsequently, residual molecules are removed, which is actually before the wafers are bonded to each other.

Thus, in either case there is no intermediate layer between the wafers that are bonded to each other, only covalent bonds formed by the condensation of hydroxide groups.

The combination of the two solids by the preheated smooth surface is not described in the previously published application.

Surface hydrophilic itself is hydrophobized by reaction with, for example, alkyltrialkoxysilanes.

This immobilized bonding of the treated surface then occurs as a result of the physical entanglement of the long chain alkyl radicals. Covalent bonds cannot be formed due to lack of reactivity of alkyl radicals.

In another known method, Langmuir-Blodgett technology is used to apply the surface, but this is a complex and expensive process.

It is an object of the present invention to provide a more advantageous method of joining solids to one another by covalent bonds at relatively low temperatures.

1 shows an example of the bond of the invention when bis (alkoxysilylpropyl) disulfane is used.

The present invention provides a method of joining virtually smooth surfaces of two solids to each other using an organosilicon compound, wherein the method attaches a monomolecular layer of the compound of formula 1 to the surfaces of two solids. It is characterized by applying.

S _y [(CH ₂ ) _x Si (OR) ₃ ] ₂

In the above formula,

R is methyl, ethyl or propyl, in particular methyl or ethyl,

x is 1, 2, 3 or 4, especially 3,

y is 2 to 6, especially 2;

Covalent bonds occur to molecules of the compound of formula 1, for example, to the hydroxide group on the silicon surface. The solid surfaces are then contacted with each other.

First, Van der Waals interactions occur between organic molecules immobilized on the wafer facing in opposite directions. During subsequent heat treatment of about 170 ° C., the S—S bonds are broken down by the formation of highly reactive species (free radicals) with secondary electrons.

Now when two free radicals bound to the oppositely facing wafer surface react with each other, covalent bonds are formed at the interface (FIG. 1). Interfacial energy is generated (20 to 300-400 mJ / m 2).

In this way such a surface can be covalently bonded by a compound according to formula 1 present in a monolayer on the surface of the substrate. Covalent bonds are significantly more stable than van der Waals interactions or hydrogen bridge bonds described in the prior art.

The process according to the invention is particularly suitable for hydrophilic surfaces. Hydrophilic surfaces are optionally produced, for example, by treatment with a mixture of H ₂ SO ₄ and H ₂ O ₂ solutions. In a preferred embodiment one of the compounds according to formula 1 is applied to the substrate (surface) from a dilute solution of the compound in an organic solvent. The concentration of the compound according to formula 1 in the solution is> 0 to ≤10 ^-3 mol / l.

Suitable solvents include saturated cyclic, branched and straight chain and aromatic hydrocarbons such as toluene and hexane. It is understood that the solids treated according to the invention include in particular those consisting of silicon, silicon dioxide, sapphire (Al ₂ O ₃ ), gallium arsenide (GaAs) or gold. These solids (substrates) have a native oxide layer (Si, SiO ₂ , Al ₂ O ₃ , GaAs) or are coated with an oxide layer, for example Y-Ba-Cu-O, and then react like a hydrophilic Si surface can do.

The method is generally carried out as follows.

Suitable substrates are exposed to a solution of about 10 ⁻³ moles of formula 1 in toluene in an inert gas atmosphere. After about 5 hours the substrate is removed from the solution, washed sequentially with toluene, acetone and methanol and then treated twice in methanol in an ultrasonic bath for 2 minutes.

The solvent residue is then removed from the wafer by mild heating while simultaneously centrifuging (3000 r.p.m., 5 minutes). The wafer is now rinsed thoroughly with ultrapure water to remove dust particles, recentrifuged to dryness, and contacted with each other. Van der Waals forces occur spontaneously between immobilized organic molecules on the wafer side facing in opposite directions. The wafer pair is then heated on a heating plate at 170 ° C. for 5 minutes and then cooled to room temperature. After this treatment, covalent bonds are formed between the wafers.

In an advantageous embodiment the compound according to formula 1 is converted to the gas phase and then attached to the monomolecular layer on the surface.

The application process from the gas phase can be carried out, for example, in a desiccator. For this purpose, the desiccator containing the compound of formula 1 as well as the substrate is evacuated to a pressure of several mbar. Then the line to the vacuum pump is cut off. After 5 hours the substrate can be coated with a monolayer of organic sulfide and contacted as described above.

Reactions in embodiments of the invention can be initiated using UV light.

The invention also provides a structural element produced by the method comprising two solid layers whose surfaces are covalently bonded to each other via a monomolecular layer formed on the surface by a compound according to formula (1).

Example

Comparing the composite solid (Si) produced according to the prior art (LB film) and that produced according to the method according to the invention (disulfane Si 266), the following results were obtained:

parameter LB-Film (Adv. Mater. 1997) Si 266 (DEGUSSA) ML at the interface 4 2 Thickness at the interface About 8nm About 1nm Bond strength at RT 100 to 350 mJ / ㎡ About 20mJ / ㎡ Bond strength after heat treatment T decomposes above 60 ℃ 300 to 400 mJ / m2 (170 ° C) WW substrate / film Van der baals Covalent coupling WW ML / ML Van der Baals + Tangled Covalent coupling Attachment method Langmuir-Blodget Adsorption from solution ML: monolayer RT: room temperature WW: interaction

The binding energy can be increased by optimizing the reaction parameters: Si 266-20 ° C: 50-80 mJ / m 2; Si 266-160 ° C (3min): 400-500mJ / m2 or UV-Licht (365nm, 7min): 750mJ / m2

Si 69-20 ° C .: 60-90 mJ / m 2; Si 69-80 ° C. (3 min) or 20 ° C. (10-14 days): 400 mJ / m 2

The reaction between two materials coated with a monolayer of Si 266 can be initiated by irradiation with UV light (365 nm, 7 min). In order to irradiate with UV light, the Si disk is replaced with a quartz wafer.

Using the method of the present invention, the solids can be bonded to each other by covalent bonds at relatively low temperatures.

Claims (8)

A method of bonding a substantially smooth surface of two solids using an organosilicon compound, characterized by applying a monomolecular layer of the compound of formula 1 to the surfaces of two solids and then contacting the solids surface. Formula 1 S _y [(CH ₂ ) _x Si (OR) ₃ ] ₂ In the above formula, R is methyl, ethyl or propyl, x is 1, 2, 3 or 4, y is 2 to 6; A process according to claim 1, wherein bis (alkoxysilylpropyl) disulfane is used as the compound of formula (I). The method according to claim 1 or 2, characterized in that a solid with a hydrophilic surface is used. The process according to claim 1, wherein the compound according to formula 1 is used in the form of a solution having a concentration of> 0 to 10 ⁻³ mol / l. The process according to claim 1, wherein the process is carried out at a temperature of 10 to 30 ° C. and optionally at a temperature of 150 to 200 ° C. 6. A process according to claim 1 wherein the compound according to formula 1 is applied from the vapor phase. The method of claim 1 wherein the reaction is initiated using UV light. A composite structural element, in particular for microelectronics or micromechanics, comprising two solids whose surfaces are bonded to one another by a monolayer of molecules formed by a compound of formula (1).